The day widely regarded as the Super Bowl of online sales ca-chinged its way to a record day for retailers, including Walmart, and marked a shift in shopping preferences as smartphones and tablets drove nearly a third of traffic — and for some retailers, more than half.

The numbers are mind-boggling. As of 6 p.m. Eastern time, overall sales were up 17.5% on Cyber Monday compared with last year, says IBM Digital Analytics Benchmark. Another e-commerce research company, Custora Pulse, found online sales up almost 19%. The numbers may shift even higher as people continue to shop through the evening.

This year, "Cyber Monday is well on its way to being the biggest online shopping day in history," says Custora CEO Corey Pierson.

Walmart.com expects to register its biggest Cyber Monday yet, coming off a record Black Friday weekend online. Heading into the afternoon Monday, the website had already sold out of PlayStation 4 and Xbox One consoles, which weren't even discounted. Significant savings on TVs and tablets also encouraged shopping, says Joel Anderson, CEO of Walmart.com.

"There's no way ... that it won't finish as our biggest Cyber Monday ever," he says.

Anderson expects that more than half of Monday's online traffic will have come from mobile devices. On Black Friday, mobile accounted for 55% of traffic to Walmart.com.

"I think 2013 will be remembered as the year online went mobile," Anderson says.

Overall, 29.4% of online traffic was mobile, according to data from IBM Digital Analytics Benchmark

That's up 61% from last year. Shoppers used smartphones to browse but were more likely to make purchases on a tablet, the data show.

And 2013 is the year the term "Cyber Monday" got a face-lift. Retailers from Target to Sears are fast evolving Cyber Monday into something more akin to Cyber Week, as online deals stretch well beyond Monday.

"You'll see retailers get incrementally more aggressive with promotions as the week goes on," says Brian Sozzi, CEO at Belus Capital Advisors.

As of 6 p.m. ET, sales on Amazon were up 44.3% over last year, while sales at eBay were up 32.1%, according to ChannelAdvisor, which tracks third-party sellers on eBay and Amazon. The online marketplaces are doing well in part because shoppers are heading to those sites to find popular products that are selling out at regular retailers, says Scot Wingo, CEO of ChannelAdvisor.

Flash-sale site Rue La La started Cyber Monday deals on Sunday with a "Cyberthon" that drew more than 350,000 people to the site. The sale promoted as much as 80% off on more than 150 brands. On Monday, sales of Cole Haan and Pandora Jewelry merchandise were strongest, CEO Steve Davis says.